It is my experience that old content is itself a form of new content. It's diverse content. Sick of dailies and LFR? Here's something else to do. Thanks to transmog, everything is of the latest tier, of appearance.

That's merely the situation for a normal player. A nostalgist gets a bonus! With every expansion there is new content. But who cares about that? They do, but I'll explain why later. First, new content means that there is old content. Old content that "no one does anymore". Old, unused, ripe for nostalgia. Get the clock ticking on forgetting the thousandth wipe and on remembering that first kill. Ah... that's the stuff.

The new content, thanks to the typical vertical progression systems, means new power. I couldn't solo Forge of Souls at 80. But I could at 85. At 90 it became easy. New content makes the old content more accessible, at least in terms of how easily the content will kill you.

On the other hand, it reduces the pool of players. All the people who want loot and rewards are gone off to purpler pastures. And good riddance! The people left are fellow nostalgists, a little community of oddities.

Taken together, there is a rolling effect. Each expansion makes more old content and each time the power level of players increases, so to does the ability of players to run old content.